A transmission system

ABSTRACT

A transmission system includes a power take-off for driving an implement and a hydrostatic unit for transmitting power from the engine to the power take-off. The hydrostatic unit includes a first hydraulic power unit having a first connection line and a second connection line, and a second hydraulic power unit having a first connection line and a second connection line. The hydrostatic unit also includes a valve for connecting the first hydraulic power unit and the second hydraulic power unit, the valve being positionable at least in a first position, in a second position and in a neutral position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a US National Stage filing of InternationalApplication Serial No. PCT/EP2017/051443 entitled “A TRANSMISSIONSYSTEM,” filed Jan. 24, 2017, which claims priority to ItalianApplication Serial No. 102016000006774, filed Jan. 25, 2016, each ofwhich is incorporated by reference herein in its entirety for allpurposes.

The invention relates to a transmission system for transmitting powergenerated by an engine to a power take-off (PTO) of a vehicle,particularly an industrial or agricultural vehicle such as a tractor.

Known tractors comprise an internal combustion engine and a transmissionsystem for transmitting power from the engine to wheels or tracks of thetractor. Downstream of the transmission system, a power-take off isnormally provided. One or more implements can be connected, if desired,to the power take-off so as to receive power from the engine through thetransmission system.

In known tractors, the power take-off is connected to the engine bymeans of a transmission consisting only of mechanical gears. As aconsequence, the speed of an output shaft of the power take-off isalways proportional to the engine speed.

However, in many cases it is desirable that the speed of the outputshaft of the power take-off can be made independent of the engine speed,and in particular that the speed of the output shaft of the powertake-off can be continuously varied independently of the engine speed.

A number of solutions have been thought in order to obtain this result,as disclosed in WO 2012/110615. In particular, some of these solutionsuse a hydrostatic unit interposed between the engine and the powertake-off in order to make the speed at the power take-off independent ofthe engine speed.

However, these solutions have the drawback that they require consistentconstructional changes to be fitted on existing vehicles and involve asignificant increase in dimensions of the rear part of the tractor.

An object of the invention is to improve known transmission systems fortransmitting power generated by an engine to a power take-off of avehicle, particularly an industrial or agricultural vehicle such as atractor.

A further object is to provide a transmission system for transmittingpower generated by an engine to a power take-off of a vehicle, whichdoes not involve a significant increase in dimensions if compared toknown transmission systems.

Another object is to provide a transmission system for transmittingpower generated by an engine to a power take-off of a vehicle, whichdoes not require excessive constructional changes to the vehicle ifcompared to known transmission systems.

A further object is to provide a transmission system for transmittingpower generated by an engine to a power take-off of a vehicle, in whichthe output speed is not proportional to the engine speed.

According to the invention, there is provided a transmission system fortransmitting power generated by an engine of a vehicle, comprising apower take-off for driving an implement and a hydrostatic unit fortransmitting power from the engine to the power take-off, wherein thehydrostatic unit comprises:

-   -   a first hydraulic power unit having a first connection line and        a second connection line;    -   a second hydraulic power unit having a first connection line and        a second connection line;    -   a valve for connecting the first hydraulic power unit and the        second hydraulic power unit, the valve being positionable at        least in a first position, in a second position and in a neutral        position, whereby:    -   in said first position, the first connection line and the second        connection line of the first hydraulic power unit are connected        respectively to the first connection line and to the second        connection line of the second hydraulic power unit,    -   in said second position, the first connection line and the        second connection line of the first hydraulic power unit are        connected respectively to the second connection line and to the        first connection line of the second hydraulic power unit,    -   in said neutral position, the first hydraulic power unit is        disconnected from the second hydraulic power unit.

Owing to the invention, both the first hydraulic power unit and thesecond hydraulic power unit can act either as a hydraulic pump or as ahydraulic motor.

Thus, the first hydraulic power unit can be a variable displacement pumphaving a displacement which can vary between zero and a maximum value,so as to provide a hydraulic ratio that can range from zero to apositive upper threshold. In order to obtain negative hydraulic ratios,the valve is switched to the second position, so that the pump works asa motor.

As a consequence, the first hydraulic pump can be a variabledisplacement piston pump, for example a radial piston pump having areduced eccentricity.

This makes it possible to obtain a transmission system having a compactand simple structure, which can be installed on existing vehicles inplace of known transmission systems without requiring consistentconstructional changes.

In an embodiment, a planetary gear train can be provided upstream of thepower take-off.

Owing to this embodiment, power generated by the engine is combined withpower generated by, or absorbed by, the hydrostatic unit, therebyallowing the power take-off to be driven at a speed which iscontinuously variable between a minimum value and a maximum value.

In an embodiment, the hydraulic pump is so controlled as to setdisplacement of the hydraulic pump to a nil value, when displacement ofthe hydraulic pump drops until reaching a preset minimum value.

Similarly, the hydraulic pump is so controlled that, when it isrequested that displacement of the hydraulic pump increases from zero toa positive value, displacement of the hydraulic pump is kept to a nilvalue, until when a preset minimum displacement is required.

This makes the transmission system particularly stable.

In an embodiment, the valve is so controlled that, if it has to passfrom the first position to the second position at a preset value ofengine speed, the valve is displaced to the second position at an enginespeed greater than said preset value.

Similarly, if the valve has to pass from the second position to thefirst position at said preset value of the engine speed, the valve isswitched to the first position at an engine speed lower than said presetvalue.

A cycle of hysteresis is thus generated, which further increases systemstability.

The invention will be better understood and carried out with referenceto the attached drawings, that show an exemplifying and non-limitingembodiment thereof, in which:

FIG. 1 is a schematic view showing a transmission system of a vehicle;

FIG. 2 is a schematic view showing a hydraulic circuit associated withthe transmission system of FIG. 1;

FIG. 3 is a diagram showing how the speed at a power take-off of thevehicle varies as a function of the speed of an engine powering thevehicle;

FIG. 4 is a view like FIG. 1, showing the transmission system in a firstworking condition;

FIG. 5 is a view like FIG. 1, showing the transmission system in asecond working condition;

FIG. 6 is a view like FIG. 1, showing the transmission system in a thirdworking condition;

FIG. 7 is a diagram showing how the speed at the power take-off,displacement of a hydraulic pump of the transmission system and positionof a valve of the transmission system vary as a function of the enginespeed;

FIG. 8 is a diagram like FIG. 7, according to an alternative embodiment.

FIG. 1 shows schematically a transmission system 1 of a vehicle,particularly an industrial or agricultural vehicle such as a tractor.The transmission system 1 allows power to be transmitted from an engine2, particularly an internal combustion engine, to a plurality of wheelsor tracks of the vehicle, that are not shown in the drawings.

The transmission system 1 further allows power to be transmitted to apower take-off 3, in order to drive one or more implements that can beconnected to the power take-off 3.

The transmission system 1 comprises a planetary gear train 4, interposedbetween the engine 2 and the power take-off 3.

The planetary gear train 4 may comprise an annular gear 9 and a sun gear10. A plurality of planet gears 11 are interposed between the annulargear 9 and the sun gear 10. The planet gears 11 are supported by acarrier 12.

The transmission system 1 further comprises a hydrostatic unit 5,arranged between the engine 2 and the power take-off 3. The hydrostaticunit 5 comprises a first hydraulic power unit, particularly conformed asa hydraulic pump 6, and a second hydraulic power unit, particularlyconformed as a hydraulic motor 7.

The transmission system 1 comprises a first input shaft 8, that iscoupled to the engine 2, for example by means of a known joint, so as tobe driven by the engine 2.

The first input shaft 8 is couplable to the planetary gear train 4, forexample by means of a clutch 13. In particular, the first input shaft 8is couplable to the annular gear 9 of the planetary gear train 4, sothat the engine 2 may drive the annular gear 9.

The transmission system 1 further comprises a second input shaft 14,that can be connected to the hydraulic motor 7. The second input shaft14 is couplable to the sun gear 10 of the planetary gear train 4, sothat the hydraulic motor 7 may drive the sun gear 10. In particular, thesecond input shaft 14 can be directly coupled to the sun gear 10.

The first input shaft 8 and the second input shaft 14 can be coaxial. Inparticular, the first input shaft 8 and the second input shaft 14 can bearranged one inside the other. In the example shown, the first inputshaft 8 is arranged inside the second input shaft 14, which is hollow. Aparticularly compact structure can therefore be obtained.

The transmission system 1 further comprises an output shaft 15 that canbe coupled to the power take-off 3. The output shaft 15 allows the powertake-off 3 to be connected to the planetary gear train 4. To this end,the carrier 12 can support a first gear 16, which meshes with a secondgear 17 fixed relative to the output shaft 15. The first gear 16 and thesecond gear 17 act as a gear assembly connecting the planetary geartrain 4 to the output shaft 15. Owing to the above mentioned gearassembly, the desired transmission ratio can be obtained at the outputshaft 15.

The hydraulic pump 6 has a shaft 18 which is connected to the outputshaft 15. The shaft 18 is fixed relative to the output shaft 15. In theexample shown, the shaft 18 is coaxial with the output shaft 15.

In an embodiment, the output shaft 15 can be integral with the shaft 18,i.e. the output shaft 15 and the shaft 18 can be one and the same shaft.

In operation, the clutch 13 is engaged when it is desired to transmitpower to the power take-off 3. Through the clutch 13, power istransmitted from the first input shaft 8 (i.e. from the engine 2) to theannular gear 9 of the planetary gear train 4. The planetary gear train 4also receives power from the hydraulic motor 7, through the second inputshaft 14 driving the sun gear 10. The motion of the planet gears 11derives from the combination of rotations transmitted by the annulargear 9 and the sun gear 10. From the planet gears 11, motion istransmitted to the second gear 17 through the carrier 12 and the firstgear 16. The output shaft 15, which is fixed relative to the second gear17, is thus rotatingly driven. The output shaft 15 in turn transmitsmotion to the power take-off 3.

The transmission system 1 is a continuously variable transmission (CVT)that allows the output speed to be continuously varied between a minimumvalue and a maximum value.

Owing to the transmission system 1, the speed of the output shaft 15,i.e. the speed at the power take-off 3, is no longer proportional to theengine speed.

Both the hydraulic pump 6 and the hydraulic motor 7 can be of the radialpiston type. Thus, the hydrostatic unit 5 can have a particularlycompact structure, because radial piston pumps and motors have reduceddimensions if compared to other kinds of pumps and motors.

The hydraulic pump 6 can be a variable displacement pump. In particular,displacement of the hydraulic pump 6 can be varied between zero and apositive maximum value.

The hydraulic motor 7 can be a fixed displacement motor.

In order to obtain negative transmission ratios of the hydrostatic unit,the hydraulic pump 6 and the hydraulic motor 7 can be included in thecircuit shown in FIG. 2.

The hydraulic pump 6 has a first connection line 19 and a secondconnection line 20 which allow the hydraulic pump 6 to be connected toother components of the circuit. Similarly, the hydraulic motor 7 has afirst connection line 21 and a second connection line 22.

A directional control valve 23 or cross-valve is interposed between thehydraulic pump 6 and the hydraulic motor 7. The directional controlvalve 23 may be a 4-way 3-position valve.

The directional control valve 23 has a distributor element or spool thatis positionable in a first position P1 or direct flow position, in whichthe first connection line 21 of the hydraulic motor 7 is connected tothe first connection line 19 of the hydraulic pump 6, and the secondconnection line 22 of the hydraulic motor 7 is connected to the secondconnection line 20 of the hydraulic pump 6. In the first position P1,the hydraulic pump 6 receives mechanical energy from the hydraulic motor7 and converts it into hydraulic energy. In other words, in the firstposition P1 the hydraulic pump 6 acts as a pump and is driven by thehydraulic motor 7, which works as a motor.

The distributor element of the directional control valve 23 further hasa second position P2 or crossed flow position, in which the firstconnection line 21 of the hydraulic motor 7 is connected to the secondconnection line 20 of the hydraulic pump 6, and the second connectionline 22 of the hydraulic motor 7 is connected to the first connectionline 19 of the hydraulic pump 6. In the second position P2, thehydraulic pump 6 transmits mechanical energy to the hydraulic motor 7which converts it into hydraulic energy. In other words, in the secondposition P2 the hydraulic pump 6 acts as a motor and drives thehydraulic motor 7, which works as a pump.

Finally, the distributor element of the directional control valve 23 canbe positioned in a central position P3 or neutral position in which thehydraulic pump 6 is isolated from the hydraulic motor 7. In particular,in the central position P3 the first connection line 21 of the hydraulicmotor 7 is connected to the second connection line 22 of the hydraulicmotor 7. Furthermore, in the central position P3 the connection lines19, 20 of the hydraulic pump 6 are both closed and are not connected toany other component.

In order to minimize pressure drop through the directional control valve23, the distributor element of the directional control valve 23 hasrelatively large dimensions. A first piloting valve 24 and a secondpiloting valve 25 are therefore provided to control position of thedistributor element of the directional control valve 23.

The first piloting valve 24 and the second piloting valve 25 areconnectable by means of an input line 26 to a supply circuit of thevehicle which supplies pressurized fluid in order to operate thepiloting valves 24, 25. The supply circuit to which the input line 26 isconnectable can comprise a boost line of the vehicle, particularly incase the latter is a tractor.

The input line 26 is connectable to the supply circuit by means of avalve 27 which can be of the on-off type. In other words, the valve 27can be positioned in a first configuration in which the input line 26 isin direct fluid communication with the supply circuit, or in a secondconfiguration in which fluid cannot flow from the supply circuit towardsthe input line 26.

The valve 27 can be positioned in the second configuration when thepower take-off 3 is not working. By so doing, when the power take-off 3is not working the hydrostatic unit 5 can be isolated from the supplycircuit, thereby allowing power to be saved.

The circuit shown in FIG. 2 further comprises a first position sensor 28and a second position sensor 29 associated with the directional controlvalve 23. The first position sensor 28 and the second position sensor 29have the purpose of determining the position of the distributor elementof the directional control valve 23, in order to establish whether thelatter is in the first position P₁, in the second position P₂ or in thecentral position P₃. The position sensors 28, 29 can be proximitysensors.

A first pressure control valve 31 and a second pressure control valve 32are provided to control pressure along the first connection line 19 andthe second connection line 20 of the hydraulic pump 6. In particular,the first pressure control valve 31 and the second pressure controlvalve 32 can be located along a joining line 30 which joins the firstconnection line 19 and the second connection line 20 of the hydraulicpump 6 to one another. The first pressure control valve 31 and thesecond pressure control valve 32 are configured to reach an openposition when pressure in the first connection line 19 and in the secondconnection line 20 overcomes a preset upper threshold. When pressure inthe first connection line 19 or in the second connection line 20 reachesthe preset upper threshold, the first pressure control valve 31 orrespectively the second pressure control valve 32 connect the respectiveconnection line to a discharge line 33.

A relief valve 34 can be provided to limit pressure in the fluid whichenters the circuit through the valve 27, particularly fluid coming fromthe boost line of the tractor. The relief valve 34 can also limitpossible pressure peaks which might arise in the circuit duringoperation.

In an alternative embodiment, the discharge line 33 can be closed andthe relief valve 34 can be used to keep pressure in the first connectionline 19 and in the second connection line 20 below a preset upperthreshold.

A pressure sensor 35 is provided to measure pressure of the fluidentering the circuit through the valve 27, particularly fluid comingfrom the boost line of the tractor. A further pressure sensor 36 allowsto monitor the maximum pressure in the circuit. Signals coming from thepressure sensor 35 and from the further pressure sensor 36 can beprocessed for diagnostic purposes.

A flashing valve 37 is interposed between the first connection line 19and the second connection line 20 of the hydraulic pump 6. The flashingvalve 37 allows the line having the lowest pressure (selected frombetween the first connection line 19 and the second connection line 20)to be connected to a discharging tank 38. Thus, it is avoided that thefluid reaches an excessive temperature inside the circuit, so that thefluid can perform a temperature conditioning function.

A pressure reduction valve 39 can be located downstream of the flashingvalve 37 along a conduit 40 which connects the flashing valve 37 to thedischarging tank 38.

A portion of fluid passing through the conduit 40 may be deviated into alubricating conduit 41, and be subsequently used for lubricating thetoothed wheels of the planetary gear train 4. An orifice 42 is providedalong the lubricating conduit 41. A check valve 43 is located along theconduit 40 downstream of the lubricating conduit 41, in order to keepconstant pressure of the lubricating fluid.

A pump control valve 44 is provided to control displacement of thehydraulic pump 6. The input to the pump control valve 44 is the highestpressure in the circuit shown in FIG. 2, which enters the pump controlvalve 44 for example by means of a shuttle valve which is not shown. Ifthe pump control valve 44 is not activated or if no fluid pressure isavailable, then the hydraulic pump 6 stays in a zero displacementcondition.

Two sensors which are not shown, particularly angle sensors, may beprovided to detect displacement of the hydraulic pump 6.

Along the first connection line 19 and the second connection line 20 ofthe hydraulic pump 6, respective joining ports H can be defined, forconnecting the first connection line 19 and the second connection line20 with an input line 45 and an output line 46 connected to an auxiliarydistributor, for example a back distributor 47 of the vehicle, which cansupply hydraulic fluid and hence power.

When the directional control valve 23 is in the central position P₃, thehydraulic motor 7 is isolated from the hydraulic pump 6.

The hydraulic pump 6 is connected to the back distributor 47 and worksas a motor controlled by the back distributor 47.

The clutch 13 can be disengaged, so that the planetary gear train 4 isisolated from the hydrostatic unit 5 and from the power take-off 3.

Hence, the power take-off 3 is driven directly by the hydraulic pump 6,which as already explained works as a motor.

This functionality can be called “hydraulic direct drive functionality”.

The back distributor 47 is a proportional distributor and the flow rateof the fluid coming from the back distributor 47 can be preciselycontrolled. Hence, the speed n_(PTO) at the power take-off 3 can becontrolled as desired, provided that power at the power take-off 3 doesnot exceed the maximum power that can be supplied by the backdistributor 47, which is limited.

In particular, by using the back distributor 47 to control the powertake-off 3, the speed n_(PTO) at the power take-off 3 can be set even tovery small values, such as 2 rpm.

Furthermore, a shaft of the power take-off 3 which is intended to beconnected to an implement can be rotated either in a preset direction orin the opposite direction. In other words, it is possible to reverse thespeed n_(PTO) at the power take-off 3.

When the engine 2 is turned off, the directional control valve 23 isdisplaced into the central position P3. The hydraulic pump 6 has adisplacement equal to zero and the shaft of the power take-off 3 can beeasily rotated by hand since the resistant torque is negligible in thiscondition.

The circuit shown in FIG. 2 can be used to brake the shaft of the powertake-off 3 when the engine 2 is turned on, for safety reasons. To thisend, the directional control valve 23 is moved to the central positionP3 and the displacement of the hydraulic pump 6 is set to its maximumvalue. The clutch 13 is disengaged.

The torque generated by the hydraulic pump 6, when the latter tries torotate, is enough to brake the slipping torque transmitted by the clutch13.

It is thus ensured that the shaft of the power take-off 3 does notrotate if accidentally touched by an operator when the engine 2 isturned on and the power take-off 3 is not working.

During operation, the directional control valve 23 is either in thefirst position P₁ or in the second position P₂, in order to ensure thatfluid circulates in the hydraulic motor 7 and the latter can becontrolled. This feature of the circuit shown in FIG. 2 can be used toprotect the shaft of the power take-off 3, as well as the correspondingclutch, from the peak torque which arises during the starting phases.

Basically, when it is desired to engage the power take-off 3, thedirectional control valve 23 is set in the central position P₃. In thisposition, the hydraulic motor 7 is braked only by the hydraulicresistance provided by the flow rate of the fluid passing through theports K-K shown in FIG. 2. By properly dimensioning this flow rate, thehydraulic motor 7 can be put in rotation while avoiding transmission ofpeak torques to the power take-off 3. Then, the directional controlvalve 23 can be gradually displaced into either the first position P₁ orthe second position P₂, while keeping the displacement of the hydraulicpump 6 equal to zero. Possible pressure peaks can be cut by the pressurecontrol valves 31, 32.

By connecting the hydrostatic unit 5 to the auxiliary distributor viathe joining ports H-H, the output shaft 15 and hence the shaft of thepower take-off 3 can be driven directly by the hydraulic pump 6. To thisend, the directional control valve 23 is displaced into the centralposition P₃ and a clutch interposed between the power take-off 3 and theoutput shaft 15 is disengaged. In this configuration, power at the powertake-off 3 is limited by the available hydraulic power on the vehicle,but virtually any speed comprised between zero and an upper threshold ortarget speed of the power take-off 3 can be reached at the output shaft15. This is due to the fact that, when the hydrostatic unit 5 isconnected to the auxiliary distributor, speed of the output shaft 15 canbe controlled by either acting on displacement of the hydraulic pump 6or by means of the auxiliary distributor. In particular, when speed ofthe output shaft 15 is controlled by controlling displacement of thehydraulic pump 6, high rotation speeds at the output shaft 15 can beachieved. On the other hand, when speed of the output shaft 15 iscontrolled by means of the auxiliary distributor, low rotation speedscan be achieved at the power take-off 3, e.g. of 1 rpm. The rotationdirection of the output shaft 15 can be both clockwise orcounterclockwise. In other words, speed at the power take-off 3 can bereversed.

The vehicle having a transmission system as the one described withreference to FIGS. 1 and 2 can reach all the working points comprisedbetween the lower line LL and the upper line UL in FIG. 3, which showsthe range of speed values n_(PTO) at the power take-off 3, as a functionof the engine speed □.

In the diagram of FIG. 3, the line L0 joins the points in whichdisplacement of the hydraulic pump 6 is zero. In this situation, torquetransmitted to the hydrostatic unit 5 is zero and the pump shaft 18 isstationary. Hence, all the torque generated by the engine 2 istransmitted—apart from internal losses—to the power take-off 3.

In particular, the working point P on the line L0 is a point in whichthe engine speed is at a value □1 corresponding to the peak powersupplied by the engine 2. In this condition, the hydrostatic unit 5 isnot active and all power is transmitted to the power take-off 3. Powertransmission at the power take-off 3 is therefore at its maximumefficiency.

This situation is also shown in FIG. 4, in which the engine 2, rotatingat the speed □1 corresponding to a peak power condition, transmits powerto the first input shaft 8 as indicated by the arrows F1. The firstinput shaft 8 is thus rotated and, through the clutch 13, transmitspower to the annular gear 9 of the planetary gear train 4, as indicatedby the arrows F2.

The hydraulic pump 6 is in a condition of zero displacement, which meansthat the hydraulic pump 6 does not send any hydraulic fluid to thehydraulic motor 7. The second input shaft 14 is therefore stationary.Hence, the sun gear 10 of the planetary gear train does not rotate. Theplanet gears 11, which are driven by the annular gear 9, in turn movethe carrier 12, which transmits power to the second gear 17 through thefirst gear 16, as indicated by the arrow F3. The output shaft 15 ishence rotated at a speed □f and power is transmitted to the powertake-off 3, as indicated by the arrow F4.

In the condition of FIG. 4, power is transmitted to the power take-off 3with the highest efficiency, because the hydrostatic unit 5 is inactiveand hence there are no power losses through the hydrostatic unit 5. Thisallows the loss of power in the transmission system 1 to be minimized.

In the working points above the line L0, the directional control valve23 is generally in the first position P1, in which the first connectionline 21 of the hydraulic motor 7 is connected to the first connectionline 19 of the hydraulic pump 6, and the second connection line 22 ofthe hydraulic motor 7 is connected to the second connection line 20 ofthe hydraulic pump 6. The hydraulic motor 7 actually works as a motorand the hydraulic pump 6 works as a pump. Hence, the hydrostatic unit 5generates power which adds to the power transmitted to the planetarygear train 4 by the engine 2 and increases speed at the power take-off3. The displacement of the hydraulic pump 6 can be varied between 0 and+1, so as to keep constant or nearly constant the speed n_(PTO) at thepower take-off 3, independently of the engine speed □.

An example of a working point above the line L0 is shown in FIG. 5,which shows a situation corresponding to the peak torque generated bythe engine 2. In this situation, both the planetary gear train 4 and thehydrostatic unit 5 are active. In particular, the hydrostatic unit 5increases speed of the output shaft 15 by adding power supplied by thehydraulic motor 7 to the power supplied by the engine 2. The speed ofthe output shaft 15 can therefore be kept constant at the value □f.

In the situation of FIG. 5, the engine 2 rotates at a speed □2 andtransmits power to the first input shaft 8 and hence to the planetarygear train 4, as indicated by the arrows E1. Also the second input shaft14 rotates and in turn transmits power to the planetary gear train 4, asindicated by the arrow E2. At the planet gears 11, power transmitted tothe planetary gear train 4 by the first input shaft 8 adds up to thepower transmitted to the planetary gear train 4 by the second inputshaft 14. The planetary gear train 4 in turn transmits power to thesecond gear 17, as indicated by the arrow E3. A fraction E4 of powerfrom the planetary gear train 4 goes to the power take-off 3, whereas afurther fraction E5 of power goes to the hydraulic pump 6.

The output shaft 15 is thus rotated at the speed □f.

In the working points below the line L0, the directional control valve23 is generally in the second position P₂, in which the first connectionline 21 of the hydraulic motor 7 is connected to the second connectionline 20 of the hydraulic pump 6, and the second connection line 22 ofthe hydraulic motor 7 is connected to the first connection line 19 ofthe hydraulic pump 6. In this condition, the hydraulic pump 6 actuallyworks as a motor, whereas the hydraulic motor 7 acts as a pump. Thehydrostatic unit 5 takes away power from the power transmitted to theplanetary gear train 4 by the engine 2 and decreases speed at the powertake-off 3.

The displacement of the hydraulic pump 6 can be varied between 0 and +1,so as to keep constant or nearly constant the speed n_(PTO) at the powertake-off 3, independently of the engine speed □.

FIG. 6 shows an example of a working condition below the line L0, inparticular a situation corresponding to the rated speed of the engine 2,i.e. the maximum speed □3 of the engine 2. In this situation, thehydrostatic unit 5 decreases speed of the output shaft 15 because thehydraulic motor 7 removes a fraction of power supplied by the engine 2.Once again, the speed of the output shaft 15 can be kept constant at thevalue □f.

In the situation of FIG. 6, the engine 2 drives the first input shaft 8and transmits power to the planetary gear train 4, as indicated by thearrows G1. The hydraulic motor 7 subtracts power from the power suppliedby the engine 2, as indicated by the arrow G2. At the exit from theplanetary gear train 4, the power resulting from the two contributionsmentioned above is transmitted to the second gear 17, as indicated bythe arrow G3. The second gear 17 in turn rotates the output shaft 15 atspeed □f, thereby transmitting power to the power take-off 3, asindicated by the arrow G4.

FIGS. 5-7 therefore correspond to three different working conditions inwhich power is distributed in different ways between the power take-off3 and the hydraulic pump 7, but the speed at the power take-off 3 isalways constant at the value □f. This confirms that, owing to thetransmission system 1, speed at the power take-off 3 can be controlledindependently of the speed at the engine 2.

When there is the need to pass from a working condition above the lineL0 to a working condition below the line L0 or vice versa, thedirectional control valve 23 has to switch from the first position P₁ tothe second position P₂ or vice versa.

For example, it is supposed that the operator desires to increase theengine speed □ while keeping the speed n_(PTO) at the power take-off 3substantially constant, so as to pass from the working point Q to theworking point R in FIG. 3. Theoretically, the displacement V_(P) of thehydraulic pump 6 should decrease when moving from the working point Q tothe working point P, reach a nil value at the working point P which ison the line L0, and then increase when moving from the working point Pto the working point R. However, from a practical point of view, due tohydraulic leakages inside the hydraulic pump 6, controlling thedisplacement V_(P) of the hydraulic pump 6 near a condition of zerodisplacement is a critical issue. In particular, the system might becomeunstable when the displacement V_(P) of the hydraulic pump 6 is below apreset limit.

In order to prevent this, when there is the need to decrease thedisplacement V_(P) of the hydraulic pump 6 down to a nil value and thenincrease it again, a control unit of the vehicle controls the hydraulicpump 6 so that, when a preset minimum value V_(Pmin) of the displacementV_(P) is reached, the displacement V_(P) of the hydraulic pump 6 isautomatically set to zero, as shown in the lower part of FIG. 7.

The displacement V_(P) of the hydraulic pump 6 is kept equal to zerountil a working condition is reached, which requires a theoreticaldisplacement V_(P) of the hydraulic pump 6 equal to the preset minimumvalue V_(Pmin). At this point, the control unit sets the displacement VPof the hydraulic pump 6 at the preset minimum value V_(Pmin), as shownin the lower part of FIG. 7. Thereafter, the displacement V_(P) of thehydraulic pump 6 can be freely increased, according to the desiredworking conditions of the transmission system. If desired, thedisplacement V_(P) of the hydraulic pump 6 can be increased until apreset maximum value V_(Pmax) is reached.

As shown by the arrows in the lower part of FIG. 7, the same controllogic is used when it is desired to decrease the displacement V_(P) ofthe hydraulic pump 6 until a nil value is reached and then increase itagain.

In FIG. 3, the control logic disclosed above corresponds to a path fromthe working condition Q to the working condition P which follows thearrows along the dashed line.

As a consequence of what has been discussed above, and as shown in thelower part of FIG. 7, the displacement V_(P) of the hydraulic pump 6 isset to zero for a range of engine speed comprised between a first value□A and a second value □B.

As shown in the upper part of FIG. 7, this affects the speed n_(PTO) atthe power take-off 3, since the speed n_(PTO) is no longer constant asthe engine speed □ increases. In particular, when the displacement V_(P)of the hydraulic pump 6 is set to zero and the engine speed is equal tothe first value □A, the speed n_(PTO) at the power take-off 3 drops to avalue n_(LOW). Thereafter, the speed n_(PTO) at the power take-off 3increases until the displacement VP of the hydraulic pump 6 takes onagain a positive value, which in the present case occurs when the enginespeed is equal to the second value □B. At this point, the speed n_(PTO)at the power take-off 3, which has reached an upper value n_(UP), dropsagain to the constant value it had before setting to zero thedisplacement V_(P) of the hydraulic pump 6.

By properly choosing the minimum value V_(Pmin) at which thedisplacement V_(P) of the hydraulic pump 6 is set to zero, thevariations of the speed n_(PTO) at the power take-off 3 (i.e. thedifference between n_(LOW) and n_(UP)) can nevertheless be kept withinacceptable tolerances.

The central part of FIG. 7 shows how the directional control valve 23 iscontrolled when the displacement V_(P) of the hydraulic pump 6 is closeto zero. In particular, the position P₂₃ of the directional controlvalve 23 is shown as a function of the engine speed □.

In the range of engine speeds below the first value □A, the directionalcontrol valve 23 is in the first position P₁ (or direct flow position).On the other hand, in the range of engine speeds above the second value□B, the directional control valve 23 is in the second position P₂ (orcrossed flow position).

However, starting from a working condition in which the directionalcontrol valve 23 is in the first position P₁, the directional controlvalve 23 is not switched from the first position P₁ to the secondposition P₂ as soon as the displacement V_(P) of the hydraulic pump 6 isset to zero. The directional control valve 23 is left in the firstposition P₁ until the displacement V_(P) of the hydraulic pump 6 isagain increased from the nil value.

In particular, just before increasing the displacement V_(P) of thehydraulic pump 6 to the minimum value V_(Pmin), the directional controlvalve 23 is moved to the second position P₂, in which the directionalcontrol valve 23 remains as the displacement V_(P) of the hydraulic pump6 increases.

If, on the other hand, the directional control valve 23 is initially inthe second position P₂ and the displacement V_(P) of the hydraulic pump6 decreases down to zero and then increases again, the directionalcontrol valve 23 is not switched to the first position P₁ as soon as thedisplacement V_(P) of the hydraulic pump 6 reaches a nil value. Asindicated by the arrows pointing to the left in the central part of FIG.7, the directional control valve 23 remains in the second position P₂until when the displacement V_(P) of the hydraulic pump 6 is raised fromzero to the minimum value V_(Pmin) near the first value □_(A) of theengine speed. In particular, the directional control valve 23 isdisplaced from the second position P₂ to the first position P₁ a momentbefore the displacement V_(P) of the hydraulic pump 6 is increased tothe minimum value V_(Pmin).

The status of the directional control valve 23 undergoes therefore asort of hysteresis in the range of engine speeds comprised between □Aand □B.

This avoids instability of the system. In particular, the control logicdisclosed above prevents the directional control valve 23 from switchingrepeatedly from the first position P₁ to the second position P₂ or viceversa when the transmission system works near the point at which thedisplacement V_(P) of the hydraulic pump 6 should theoretically be equalto zero.

As shown in FIG. 8, an additional load sensing control of the speedn_(PTO) at the power take-off 3 can be provided. In other words, thedisplacement V_(P) of the hydraulic pump 6 can be properly varied inorder to obtain a slow variation of the speed n_(PTO) at the powertake-off 3, depending on the engine speed.

If, for example, there is an increase in the load on an implementattached to the power take-off 3, power required at the power take-off 3also increases. As a consequence, the engine speed □ would tend todecrease. In this case, it is possible to decrease the displacementV_(P) of the hydraulic pump 6 in a controlled manner, so that also thespeed n_(PTO) at the power take-off 3 decreases, before the engine speed□ is reduced.

This allows a better control of the engine speed to be obtained.Furthermore, the operator may have a more precise feeling about what isoccurring at the power take-off 3.

The speed n_(PTO) at the power take-off 3 can be reduced to such anextent, that the engine speed □ remains substantially constant. Thisallows fuel consumption to be optimized.

In other words, it is either possible to keep speed n_(PTO) at the powertake-off 3 nearly constant and allow the engine speed □ to decrease, orto keep the engine speed □ substantially constant and decrease speedn_(PTO) at the power take-off 3.

The control logic used for controlling displacement of the hydraulicpump 6 and position of the directional control valve 23 can also be usedin a system which does not comprise a power take-off and/or a planetarygear train.

1-9. (canceled)
 10. A transmission system for transmitting powergenerated by an engine of a vehicle, comprising: a power take-off fordriving an implement; and a hydrostatic unit for transmitting power fromthe engine to the power take-off, wherein the hydrostatic unitcomprises: a first hydraulic power unit wherein the first hydraulicpower unit is a hydraulic pump having a first connection line and asecond connection line; a second hydraulic power unit wherein the secondhydraulic power unit is a hydraulic motor having a first connection lineand a second connection line; a valve for connecting the first hydraulicpower unit and the second hydraulic power unit, the valve beingpositionable at least in a first position, in a second position and in aneutral position, wherein when the engine is turned off, the valve isdisplaced in the neutral position and displacement of the hydraulic pumpis set to zero, thereby allowing a shaft of the power take-off to berotated by hand.
 11. The transmission system according to claim 10,wherein: in the first position, the first connection line and the secondconnection line of the first hydraulic power unit are connectedrespectively to the first connection line and to the second connectionline of the second hydraulic power unit, in said second position, thefirst connection line and the second connection line of the firsthydraulic power unit are connected respectively to the second connectionline and to the first connection line of the second hydraulic powerunit, in said neutral position, the first hydraulic power unit isdisconnected from the second hydraulic power unit.
 12. The transmissionsystem according to claim 10, wherein the system further comprises aplanetary gear train and a clutch for selectively coupling the engine tothe planetary gear train, wherein when the engine is working and thepower take-off is inactive: the valve is displaced in the neutralposition, displacement of the hydraulic pump is set to a maximum value,and the clutch is disengaged.
 13. The transmission system according toclaim 10, wherein the displacement of the hydraulic pump is variablebetween zero and a maximum positive value in order to control speed atthe power take-off independently of engine speed.
 14. The transmissionsystem according to claim 13, wherein the hydraulic pump is configuredto be controlled so as to modify the displacement (V_(P)) of thehydraulic pump, to cause a speed at the power take-off to be modifiedresponsive to a load acting on the power take-off.
 15. The transmissionsystem according to claim 11, wherein, in the neutral position, thefirst connection line and the second connection line of the secondhydraulic power unit are connected to each other and are isolated fromthe first connection line and from the second connection line of thefirst hydraulic power unit.
 16. The transmission system according toclaim 10, further comprising a planetary gear train and a clutch forselectively coupling the engine to the planetary gear train.
 17. Thetransmission system according to claim 10, wherein the valve comprises adistributor element, a first piloting valve and a second piloting valvebeing provided for controlling position of the distributor element, thefirst piloting valve and the second piloting valve being connectable bymeans of an input line to a supply circuit capable of supplyingpressurized fluid in order to operate the first piloting valve and thesecond piloting valve, a further valve being provided for eitherisolating the input line from the supply circuit or connecting the inputline to the supply circuit, the further valve being so controlled as toisolate the input line from the supply circuit when the power take-offis inactive, thereby allowing power to be saved.
 18. The transmissionsystem according to claim 17, further comprising a first position sensorand a second position sensor for determining position of the distributorelement.
 19. The transmission system according to claim 10, and furthercomprising a flashing valve interposed between the first connection lineand the second connection line of the first hydraulic power unit forconnecting to a discharge tank the line having the lowest pressureselected from between the first connection line and the secondconnection line of the first hydraulic power unit.
 20. The transmissionsystem according to claim 19, and further comprising a planetary geartrain, a lubricating conduit branching from a conduit that connects theflashing valve to the discharge tank in order to deviate a portion offluid passing through the conduit towards the planetary gear train,thereby lubricating a plurality of toothed wheels of the planetary geartrain.